The field of coal treatment and upgrading to provide various fuels and particularly liquid fuels which would replace petroleum derived liquid fuels has been the subject of intense research from the 1920's in Germany through the present worldwide energy dilemma. The techniques used to recover more easily utilized fuels from raw coal are generally termed coal liquefaction. Coal liquefaction can be performed in a wide variety of processes which are performed on non-anthracitic coals, such as bituminous, sub-bituminous and lignitic coals, as well as other organic materials such as peat.
Coal liquefaction may be performed in a thermal or catalytic process. In a thermal process, heat is utilized to perform the liquefying function of the process along with any catalysis which may occur due to indigenous minerals found in the coal naturally. In a catalytic process, an added catalyst in conjunction with heat is utilized to perform the liquefying function of the process. In addition, the catalytic reaction can be performed in any number of reactors, such as a slurry phase reactor and an ebullated bed reactor.
Generally, coal liquefaction processes attempt to break weak heteroatom and carbon to carbon linking bridges in the coal molecular structure and then hydrogenate the resulting radicals to inhibit polymerization to high molecular weight structures. Hydrogen is necessary to perform this function in coal liquefaction operations which operate as a hydrogenation reaction. Hydrogen can be added to a coal liquefaction reaction in the gas phase, but those skilled in the art have found that hydrogen is best introduced into the liquefaction reaction by a hydrogen donor solvent. Such solvents must be miscible with the coal reaction products and must also reversibly hydrogenate and dehydrogenate in order that hydrogen can be loaded on the solvent, introduced into the reaction medium and donate the hydrogen to the feed coal that is liquefying. The hydrogen depleted solvent is then separated from the liquid coal product and is rehydrogenated and returned or recycled for further duty in the coal liquefaction reaction.
In recycling hydrogen donor solvent, the prior art has performed various separations and purifications of the recycle solvent in order to benefit the reaction to which the solvent is introduced. For example, U.S. Pat. No. 4,056,460 teaches the beneficial effect of recycling or adding amines and phenols as coal solvent constituents. A process is described in which phenols and amines are extracted from coal liquids and are recycled to the liquefaction zone. The patent identifies the phenolic functionalities in asphaltenes and states that although asphaltenes should not be recycled to the liquefaction reaction, after a reduction in molecular weight, the corresponding phenols are beneficial recycle solvents.
U.S. Pat. No. 4,057,484 discloses that acid-base structures in asphaltenes must be decomposed with either acid alone or caustic alone before a portion of the asphaltenes are recycled as pasting solvent for coal liquefaction.
In U.S. Pat. No. 4,081,351, a coal extract is denitrogenated before going to a catalytic cracker reaction vessel in order to protect the catalyst in the hydrocracker from nitrogenous components in the coal extract. A portion of the fractionated effluent from the catalytic cracker vessel is recycled and incorporated with petroleum solvent for utilization in a coal liquefier.
U.S. Pat. No. 4,125,452 discloses a coal liquefaction process in which a recycle solvent is separated from phenol-containing material and the phenol-free solvent is rehydrogenated before recycle to a coal liquefier.
In U.S. Pat. No. 4,133,646 it is stated that a coal liquefaction process wherein 3 to 50% of phenols are recycled as solvent is a desired goal of the patented process.
The prior art, as evidenced above, has treated the effluent and the recycled solvent of coal liquefaction processes in various ways in order to benefit the products from coal liquefaction or the liquefaction reaction itself. However, the prior art has not recognized the advantage set forth below in the present invention wherein nitrogenous and phenolic constituents of a solvent are removed prior to the solvent being introduced into a coal liquefaction reaction. In addition, it is an advantage of the present invention to not rehydrogenate the recycle solvent before introduction into the coal liquefaction reaction in order to avoid the production of phenolic components by the interaction of residual ethers in the solvent and hydrogen which would be administered in a rehydrogenation step. However, rehydrogenation followed by phenol extraction could be performed. An improved liquefaction reaction with increased oil production is experienced when the denitrogenation and the dephenolation of the present invention is performed on a coal solvent, particularly in the absence of a separate rehydrogenation step where phenols could be reformed.